Aspects of small airborne passive millimetre-wave imaging systems

[摘要] ENGLISH ABSTRACT: Passive millimetre-wave (PMMW) imaging is a technique that uses radiometers to detect thermal radiationemitted and reflected by metallic and non-metallic objects. While visual and infra-red emissions are attenuatedby atmospheric constituents, PMMW emissions are transmitted, resulting in consistent contrast between differentobjects from day to night in clear weather and in low-visibility conditions to form images for a range ofsecurity and inclement weather applications.The use of a PMMW imaging system on a small unmanned aerial vehicle (UAV) offers extremely attractivepossibilities for applications such as airborne surveillance for search and rescue operations, which are oftenhindered by inclement weather making visibility poor and endangering the rescuers as the search vehicle fliesthrough the bad weather zone. The UAV would fly above the bad weather zone, with the PMMW imagingsystem detecting the thermal radiation emitted and reflected by objects in the MMW spectrum through theinclement weather. The 35GHz propagation window is chosen for the greater transmission through atmosphericconstituents.The design of the PMMW imaging system is severely limited by the size of the UAV, particularly in theinability to incorporate any form of optical or mechanical scanning antenna. A possible solution is a long, thinantenna array fitted under the wings of the UAV. Such an antenna has a narrow, high gain, frequency-scannedbeam along the plane perpendicular to the flight path, but a very broad beam along the plane of the flight pathblurs the image, making it difficult to accurately determine the position of an object or to differentiate betweenobjects situated along the plane of the flight path.This dissertation proposes a technique of image reconstruction based on the Kalman filter, a recursive filterthat uses feedback control to estimate the state of a partially observed non-stationary stochastic process, toreconstruct an accurate image of the target area from such a detected signal. It is shown that given a simulatedtarget area, populated with an arbitrary number of objects, the Kalman filter is able to successfully reconstructthe image using the measured antenna pattern to model the scanning process and reverse the blurring effect

[发布机构] Stellenbosch University